Accompanying the increasingly higher levels of integration of semiconductor devices in the semiconductor industry in recent years, there is a growing need for fine patterns that exceed the transfer limitations of conventional photolithography methods using ultraviolet light. Extreme ultraviolet (EUV) lithography is considered to be promising as an exposure technology that uses EUV light to enable the formation of such fine patterns. Here, EUV light refers to light in the soft X-ray region or vacuum ultraviolet region, and more specifically, light having a wavelength of about 0.2 nm to 100 nm. Reflective masks have been proposed as transfer masks for use in this EUV lithography. Such reflective masks have a multilayer reflective film that reflects exposure light formed on a substrate, and an absorber film that absorbs exposure light is formed in a pattern shape on the multilayer reflective film.
The reflective mask is manufactured from a reflective mask blank having a substrate, a multilayer reflective film formed on the substrate, and an absorber film formed on the multilayer reflective film, by forming an absorber film pattern by photolithography and the like.
As has been described above, due to the growing demand for miniaturization in the lithography process, significant problems are being encountered in the lithography process. One of these is the problem relating to defect information of substrates with a multilayer reflective film used in the lithography process.
Substrates with a multilayer reflective film are being required to have even higher smoothness from the viewpoints of improving defect quality accompanying the miniaturization of patterns in recent years and the optical properties required of transfer masks. The multilayer reflective film is formed by alternately laminating a high refractive index layer and a low refractive index layer on the surface of a mask blank substrate. Each of these layers is typically formed by sputtering using sputtering targets composed of the materials that form these layers.
Ion beam sputtering is preferably carried out as a sputtering technique from the viewpoints of the multilayer reflective film being resistant to contamination by impurities since there is no need to form plasma by electrical discharge, and the comparative ease with which conditions can be set due to the use of an independent ion source, while from the viewpoints of smoothness and surface uniformity of each of the layers formed, a high refractive index layer and a low refractive index layer are deposited by allowing sputtered particles to reach the substrate at a large angle relative to the normal of the main surface of the mask blank substrate (line vertical to the main surface), or in other words, at an angle that is diagonal or nearly parallel to the main surface of the substrate.
Patent Literature 1 describes an example of a technology for manufacturing a substrate with a multilayer reflective film using this type of method wherein, when depositing a multilayer reflective film of a reflective mask blank for EUV lithography on a substrate, ion beam sputtering is carried out by maintaining the absolute value of an angle α formed between the normal of the substrate and sputtered particles entering the substrate such that 35 degrees≦α≦80 degrees while rotating the substrate around its central axis.